(1) Field of the Invention
The invention relates to an apparatus, and method for establishing the position of a moving platform.
(2) Description of the Related Art
A positioning reference system (PRS) is a component of an elevator control system that provides fast and accurate position measurement of an elevator car in a hoistway. Some PRSs make use of vision based systems, such as charge couple devices (CCD) attached to a moving platform, in conjunction with visual indicators attached to fixed positions along a hoistway. Under such a scenario, the vision system observes the visual indicators, typically passive reflectors, identifies the location of the visual indicator, and computes a position of the movable platform therefrom.
Unfortunately, the signal to noise (S/N) ratio of CCD based vision systems employing passive reflectors can be substantially degraded due to opaque materials in the air, on the CCD lens, and/or on the passive reflectors. Such a degraded S/N ratio can lead to degradation of positioning performance of the CCD based PRSs in a worst case scenario. Use of a high intensity light illumination source for the visual indicator can form a satisfactory solution for preventing such performance degradation. Another solution involves the utilization of active reflectors, specifically reflectors which do not necessarily passively reflect light but which actively perform as a light source and are comprised, for example, of light omitting diodes (LEDs) instead of passive reflectors as noted above. Use of active reflectors is often times preferable as an active reflector can provide the necessary signal to noise ratio by controlling the intensity of the light emitted. Specifically in instances wherein power is available where the active reflectors are to be located, use of active reflectors forms methodology by which the S/N ratio may be increased to suitable levels for allowing accurate positioning of the movable platform.
However, there exist several critical problems associated with active reflector based CCD systems. First, the lifespan of an active reflector is limited as the longest lifespan of existing light sources is at most ten years. Increases in the ten year lifespan may be achieved by turning on and off the light sources comprising the active reflectors such that each light source is illuminated for only a few milliseconds out of every ten milliseconds. However, in the case of active reflectors, the only opportunity to turn off the active reflectors is when the elevator is not serving passengers, a duration of which cannot be clearly ascertained. In addition, turning on and off active reflectors in the above-noted fashion requires additional control/signal wirings which in turn increase the cost of installation.
Second, in order to endow a PRS with the capability of not requiring a correction run, the active reflectors are preferably encoded. Such encoding usually results in higher cost and less robust operation. These facts, coupled with the limited lifespan of active reflectors, leads to a high maintenance cost as well as a high material and installation cost.
What is therefore needed is a PRS which incorporates active reflectors in such a manner as to allow for a substantially longer lifespan of operation, while providing low cost installation and maintenance.